A control mode of the power converter circuit (a DC/DC converter or an AC/DC converter) includes a continuous mode, a discontinuous mode and a critical mode conventionally (refer to a patent document 1).
The critical mode is a mode of a border between the continuous mode and the discontinuous mode.
FIG. 19 (A) shows a buck type (a step-down type) DC/DC converter.
FIG. 19 (B) shows each waveform in several points of the buck type DC/DC converter working in the continuous mode.
FIG. 19 (C) shows each waveform in several points of the buck type DC/DC converter working in the discontinuous mode.
FIG. 19 (D) shows each waveform in several points of the buck type DC/DC converter working in the critical mode.
‘Vs’ is a switch voltage, ‘is’ is a switch current, ‘STON’ is a switch drive signal, ‘iL’ is an inductor current and ‘VL’ is an inductor voltage in FIG. 19 (B), FIG. 19 (C) and FIG. 19 (D).
A power factor in the critical mode (FIG. 19 (D)) is usually higher than a power factor in the continuous mode (FIG. 19 (B)) and a power factor in the discontinuous mode (FIG. 19 (C)).
FIG. 20 shows a conventional power conversion system to work in the critical mode, which comprises a control device 8 and a power converter circuit 9.
In FIG. 20, the power converter circuit 9 is a buck type DC/DC converter. The power converter circuit 9 comprises a DC power supply 911, a switch 912 (a transistor), a flywheel diode 913 (DF), an inductor 914 (L) and a capacitor 915 (CO).
A power input terminal of the switch 912 is connected to a positive terminal of the DC power supply 911.
A cathode terminal of the flywheel diode 913 (DF) is connected to a power output terminal of the switch 912, and an anode terminal is grounded.
A terminal of the inductor 914 (L) is connected to the power output terminal of the switch 912, and another terminal of the inductor 914 (L) is connected to a terminal of the capacitor 915 (CO).
Another terminal of the capacitor 915 (CO) is grounded.
A load 900 is connected to both terminals of the capacitor 915.
A resistance 916 (r0) for detecting an inductor current is connected to the inductor 914.
But, if a secondary winding is provided in the inductor 914, a resistance for detecting the inductor current can be connected to the secondary winding.
In the power converter circuit 9, energy is accumulated to the inductor 914 when the switch 912 is turned on.
On the other hand, the energy accumulated to the inductor 914 is released to the load 900 when the switch 912 is turned off.
The control device 8 includes an on-time information generation circuit 81, a zero cross detecting circuit 82 and a PWM signal generation circuit 83.
The on-time information generation circuit 81 receives a power converter circuit information INF including at least an output voltage value EO of the power converter circuit 9, and the on-time information generation circuit 81 generates the on-time information NTON of the switch 912.
An output voltage eO of the power converter circuit 9 is converted to a digital signal (the output voltage EO) by an A/D converter 801, and the output voltage EO is input to the on-time information generation circuit 81.
The on-time information generation circuit 81 is typically a PID control circuit. The on-time information generation circuit 81 can send the on-time information NTON (a numerical value) to the PWM signal generation circuit 83 for each switching cycle.
For example, the on-time information generation circuit 81 sends the on-time information NTON to the PWM signal generation circuit 83 depending on a request from the PWM signal generation circuit 83.
A voltage Vr0 occurs between two terminals of the resistance 916 (r0) serially connected to the inductor 914.
The zero cross detecting circuit 82 inputs the voltage Vr0.
The zero cross detecting circuit 82 detects a time when a current flowing through the inductor 914 (an inductor current iL) becomes zero.
The zero cross detecting circuit 82 generates and sends a zero cross detection signal ZCRSS to the PWM signal generation circuit 83 when the inductor current iL becomes zero.
When the zero cross detection signal ZCRSS is received, the PWM signal generation circuit 83 sends a turn on signal TRNON to a driver 802.
The PWM signal generation circuit 83 generates and sends a turn off signal TRNOFF to the driver 802 when a time based on the on-time information NTON passed after the PWM signal generation circuit 83 sent the turn on signal TRNON.
The driver 802 drives the switch 912 based on the switch drive signal STON.